Advocates laud intermittent fasting as a weight-loss breakthrough, and scientists have started uncovering the potential health benefits of taking measured breaks from eating, for some people. Dietitian Katrina Pace looks at the research behind fasting.
Once the preserve of religious or spiritual practice, fasting has made a mainstream appearance in recent years. At Healthy Food Guide we’re not fans of diets, where food is severely restricted in order to lose weight. Rather, we advocate long-term healthy eating that maximises our health in all ways. But weight loss is a huge challenge for many, with no single approach that suits all. Intermittent fasting offers a different way of eating that can work for some people. And it’s not just about weight loss, there are other health gains that make fasting well worth a closer look.
The origins of fasting
Fasting can be traced as far back as biblical times. Fasting is also a key aspect of the Muslim faith, with a whole month (Ramadan) being devoted to fasting during daylight hours. However, the act of religious fasting has more to do with removing food and focusing energy on prayer or devotion, than health gains.
Fasting for health started with looking at how to increase lifespan. In both animal and human studies, fasting has been shown to reduce the markers of chronic health conditions and improve features that result in longer life. In the last 15 years, fasting research has focused mainly on single days where food is restricted or removed a few times a week, rather than long periods of total fasting. What we do know is making sure you have enough nutrition overall is very important, as health gains from fasting may be offset by nutrient deficiencies causing poor health, such as osteoporosis.
The science so far
So what are the health benefits of fasting? Apart from weight loss, intermittent fasting has been associated with reducing inflammation, altering gene expression, improving cardiovascular disease, affecting how cells react to chemotherapy, improving diabetes control and changing our gut bacteria. While most human studies are still short term, at up to six months, the current research indicates intermittent fasting has comparable effects to standard energy restricted diets on weight loss and inflammation, and may be even better for managing blood glucose.
Different types of fasting
The 5:2 style of intermittent fasting is the most popular type of non-religious fasting. Five days of the week you eat as you normally would, and two days a week you cut down to about a quarter of what you’d usually eat.
This type of fasting is mainly promoted for weight loss, but for those who want the health benefits of fasting but don’t want to lose weight, non-fasting days involve eating around 25 per cent more than usual, to help maintain weight.
Two types of 5:2 style ways of fasting exist. Dietitian Michelle Harvie and oncology professor Tony Howell, in their research-based book The 2 Day Diet, advise three meals on fasting days, whereas journalists Michael Mosely and Mimi Spencer, in their book The Fast Diet, advise two meals. Which is right?
Research suggests they have similar benefits, and that timing of the meals has little impact.
So three meals a day or two meals doesn’t matter, it’s which works best for you.
‘Time-restricted eating’ is another approach to fasting that’s been shown to be important for health. Energy intake isn’t changed at all, but all your food is eaten within a 7-12 hour window during the day (see How to time-restrict eat, below).
Fasting and weight
Intermittent fasting can help people lose weight if they need to. But be warned, claims the weight will just drop off can be taken with a grain of salt. Repeated research has shown that intermittent fasting, when used for the purpose of weight loss, only helps people lose the same amount of weight as any other healthy eating-based weight-loss diet.
Most weight-loss diets cut your energy intake by about 25 per cent each day.
Intermittent fasting cuts intake by 75 per cent twice a week. Over seven days, this works out about the same amount (25 per cent) of energy restriction.
On most diets you can expect to lose about 0.5kg each week. Not The Biggest Loser numbers by any stretch of the imagination, but solid, regular losses. So, the bottom line is people who fast and don’t increase their energy intake to make up for it on non-fasting days can expect to lose weight, but not rapidly.
Just like weight-loss diets, fasting has varying results and ‘stickability’, depending on the person doing it.
A number of years ago the BBC hosted a weight-loss show similar to The Biggest Loser, called What’s the Right Diet for You? In the background were a host of doctors, dietitians and psychologists who grouped people into certain types. They hoped to show that these personality types did well on certain types of diets.
They believed people who best fit an intermittent fasting approach for weight loss tended to:
- Constantly think about food: what to have, where to get it, looking for food, grazing on food
- Be very aware of food shown to them. It catches their eye where others might not see it (or give it attention)
- Have an ‘all or nothing’ approach to dieting, and if they ‘fall off the wagon’ that’s it, all over
- Be unable to stick to diets day in, day out. They get bored, busy or it doesn’t fit with their culture or lifestyle.
What intermittent fasting seems to teach people is that it’s okay to feel hungry sometimes. Feeling hungry can make some people feel anxious as they associate it with something negative. A fear of hunger can cause problems for some people who need to lose weight, and is sometimes associated with overeating. But people who try intermittent fasting say that knowing they’ll have a certain food at a certain time relieves this fear and anxiety, and reminds them that hunger is okay sometimes.
Fasting and gut health
It seems our gut bacteria like short periods of fasting. Most of the research is in animals, but now a few human studies have shown fasting can decrease the bacteria associated with inflammation and help bacterial diversity – two important features when improving health.
It’s time-restricted eating that has most to do with changes to your gut bacteria. Your gut bacteria are affected by day-night body patterns (circadian rhythm). If your body has periods without food overnight it can help re-establish disrupted circadian rhythms, improving gut health and, therefore, overall health.
Having a decent overnight fast may be a great place to start to give your gut bacteria a helping hand. Having a 10-12-hour overnight period, say 7pm-7am where no food is consumed, is a very traditional way of eating. After all, that’s why breakfast is called ‘breakfast’: break fast.
But these days we often work and eat late, or snack late in front of the TV.
By giving our body a decent food break overnight and not snacking or eating late, we give our gut microbiota a chance to improve and also get our body’s circadian-driven metabolism time to get back to normal.
Fasting and diabetes
Telling people with diabetes to miss meals, when current guidelines are to eat regular meals, seems strange, but there’s increasing evidence that intermittent fasting may be beneficial to people with diabetes – and can be done safely. Research following Muslims who fast as part of Ramadan shows that as long as medication is properly adjusted and education is given around safe diabetes management, then fasting can be safely done by people with types 1 and 2 diabetes. Recent research in both New Zealand and Australia has also shown that intermittent fasting is a safe choice for people with all types of diabetes, as long as they have correct advice from their doctor or diabetes team on medication management.
Intermittent fasting can help people with diabetes if weight is an issue.
Weight loss in people with pre-diabetes, or with type 1 or type 2 diabetes who are overweight, can help to reduce insulin resistance and improve insulin sensitivity. Studies have suggested that intermittent fasting can reduce inflammation, preserve lean muscle mass and improve insulin sensitivity, all important factors that can improve glucose control in people with diabetes.
How to make fasting days more successful
- You don’t need to fast on the same days every week, but doing so can make it easier to get into a routine
- You might not want to fast on a day that visitors are coming
- Choose fasting days where you are busy or have a lot of appointments so you don’t have time to think too much about food
- Swap days if you have to go to a hui, tangihanga, meeting, celebration or party
- You don’t need to fast on two days together, but can if that works for you.
Make sure you drink enough
- Water is best, but tea or coffee with a splash of milk is fine. Try not to have fruit juice
- At least 8-10 glasses on fasting days will help stop you getting headaches, but avoid having too much caffeine.
How to time restrict eating
If you want to try time-restricted eating, the easiest way is to make dinner your last meal of the day. Aim to eat dinner by 7.30pm if you have breakfast around 6.30-7am.
For many of us, evening snacking has become quite normal. It’s quiet, we’re relaxing after a busy day, we’re watching TV and out come the snacks. Mindful eating habits can be a great way to help us stop snacking in the evenings and give ourselves a fasting period overnight.
- Take some time to consider if you’re snacking because you’re hungry, or for another reason? If you’re snacking because you’re bored, try picking up a hobby, such as colouring in, knitting or puzzles. Go for a walk or do some gardening in the summer months
- If you are hungry, make sure you’re having enough to eat at dinner. Protein will help fill you up, and adding lentils or beans will keep you full for longer too
- Sometimes we confuse hunger for thirst. Have a cup of tea and then ask yourself, “I can have anything I want, but do I really want this?” It will give you a chance to see if you really want to eat, and give you a chance to opt out or make a more appropriate choice.
What does an intermittent fasting day look like?
On a fasting day, women are usually advised to have about 2000kJ (500cal), and men 2500kJ (600cal). Here’s an example of what to eat on a 2000kJ (500cal) fasting day.
Remember, this is only two days a week. If you don’t want to lose weight, on non-fasting days you’ll need to eat more to compensate. If you are aiming to lose weight, be sure not to compensate by eating more than usual on non-fasting days.
Three meals a day
- Breakfast: 1 thin slice wholemeal toast, with 1 teaspoon spread and 1 teaspoon jam or Marmite
- Lunch: 2 whole-wheat crackers with 95g tin tuna in spring water (drained), with 1 cup green salad
- Evening meal: 1/2 grilled or baked chicken breast served with 1/2 cup mixed vegetables and 1/4 cup cooked brown rice.
Two meals a day
- Breakfast: 1/2 cup low-fat plain yoghurt with 1/4 cup blueberries and 1/2 medium banana
- Dinner: 185g tin tuna in spring water (drained), 1/2 can mixed beans in spring water, 2 tomatoes, 2 cups baby spinach and 1/4 cup cooked brown rice.
Add 500kJ for men
Men can have 500kJ more each day (total 2500kJ/600cal) daily. Choose one of these options:
- 140ml plain yoghurt and 1 small piece fruit
- 2 tablespoons hummus and 1 thick (10g) wedge cheese
- 1 apple and 1 tablespoon peanut butter.